1. Field of the Invention
The present invention generally relates to implantable medical devices such as implantable cardiac pacemakers and in particular to devices for automatically adjusting a sensing threshold of such medical devices.
2. Description of the Prior Art
Modern implantable stimulation devices generally include sensing capabilities, in one or two chambers of the heart. Typically, sensing of the low amplitude cardiac signals is achieved by means of sense amplifier circuits which amplifies and filters electrogram signals obtained by electrodes placed in or on the heart and which are connected by suitable leads to the implantable device. In general, the signals from the sense amplifier are applied to one input of a comparator circuit whose other input is connected to a reference source, i.e. a reference potential. Only in those cases where an electrogram signal from the sense amplifier exceeds the reference potential it will be treated as a detected cardiac depolarization event such as a P-wave (atrial intrinsic depolarization event). The reference potential may thus be referred to as a sensing threshold.
Sensing circuits of the above-mentioned type often fail to sense cardiac depolarizations since the cardiac depolarization events can result in widely different peak amplitudes, depending on e.g. patient activity, body position, and/or drugs being used, etc. Movements of the leads and noise may further restrain the detection of cardiac depolarization events. Noise sources may include environmental noise, such as 50 or 60 Hz power line noise, motion artifacts, or T-waves.
Programmable implantable devices offer a higher degree of flexibility since the physician can change the threshold potential of the comparator but, in spite of this, sensing errors will still occur due to above-mentioned reasons frequently enough to result in, for example, patient discomfort. When the threshold is set too low, the likelihood of over-sensing increases significantly, i.e. erroneous sensing of noise or undesired cardiac signals may occur. On the other hand, if the sensing threshold is set too high the likelihood of under-sensing is increased, i.e. some cardiac event will not be detected.
Furthermore, under-sensing may also cause an R-wave preceded by an atrial event, which may be, for example, a P-wave or a PAC (premature atrial contraction), to be interpreted as a PVC (premature ventricular contraction) due to the fact that the atrial event was not detected by the atrial detecting means. This phenomenon is hereinafter referred to as a false PVC. In particular, with respect to PAC's, the amplitudes of PAC's generally are significantly lower than the amplitudes of sinus induced P-waves, which entails that a PAC is almost impossible to detect using the prior art methods and apparatus for adjusting the sensing thresholds. In certain therapeutical applications it is desirable to identify PVCs, i.e. to identify the source of a PVC in order to distinguish between true PVCs and false PVCs, where a true PVC is an R-wave, without a preceding atrial event. This identification is very useful for diagnostic purposes.
Several attempts have been disclosed addressing the problems associated with under- and/or over-sensing, for example, U.S. Pat. No. 6,418,343, to Zhang et al. entitled “Method an apparatus for adjusting the sensing threshold of a cardiac rhythm management device”. The method according to U.S. Pat. No. 6,418,343 is, however, complicated and requires rather extensive calculation capabilities of the controller of the device.
Hence, there is a need for an implantable stimulation device in which the sensing thresholds in an effective and reliable way can be automatically adjusted on a continuous basis by the device itself.